[Oberon] GC of Extended Oberon now also works if MemLim = N*256

[Andreas Pirklbauer]

The scan phase of the garbage collector of Extended Oberon
now also works if MemLim is a multiple of 256. Thanks to the
uses of the system for the input on this over the past few days -
it helped arriving at a satisfactory solution quickly. The key is to
check for p < heapLim during the scan phase at the right places.